The following relates to printing systems. It finds particular application in conjunction with adjusting print uniformity for printing multiple halftones and will be described with the particular reference thereto. However, it is to be appreciated that the following is also amenable to other like applications.
Typically, within the printing system, a digital image gray level value is passed as an input to an image processing module containing functions such as the Tone Reproduction Curve (TRC) for each of the separations. The Tone Reproduction Curve modifies the input gray level value in accordance with the rules determined typically during calibration such as during gray balance as described in Mestha, “Gray Balance Control Loop for Digital Color Printing Systems,” identified below, and provides the modified gray level value to a marking engine. The Engine Response Curve (ERC) of the marking engine describes the printed density as a function of the modified gray level input provided to the marking engine by the Tone Reproduction Curve. An ERC can be constructed and updated at regular intervals during print runs while executing process control functions. One method for updating an ERC employs optical sensors to measure print density on a belt as disclosed in U.S. Pat. No. 5,471,313, identified below. Accordingly, the marking engine writes toner or ink on a print media at a density determined by the modified gray level. The System Response Curve (SRC), as used in the present application, is the relationship between the input gray level and the output print density.
To produce high quality prints, the printer is required to maintain a consistent System Response Curve over time and space. An ideal, robust marking engine would have an ERC or SRC that is stable over space and time. However, the performance of sub systems in the marking engine due to disturbances, such as various halftones, media type, and environmental zones, may vary. ERC variation over space may result in streaks, which are generally defined as variations in the print density image quality in the cross process direction. In addition, ERC variation over time may lead to page-to-page color shifts.
One approach to maintain a consistent System Response Curve is to use feedback control, in which the stability or uniformity over time is achieved by monitoring patches at selected grayscale levels. The Engine Response Curve is inferred over all halftone gray levels based on the monitored patches. The compensating Tone Reproduction Curve is derived and applied based on the inferred Engine Response Curve such that the overall System Response Curve is maintained. Likewise, stability or uniformity over space is achieved by monitoring the uniformity of strips at selected grayscale levels. The spatially dependent Engine Response Curve is inferred over all halftone gray levels based on the monitored patches. Spatially varying compensating TRC is applied based on the spatially varying ERC. However, typically, the marking engine prints more than one halftone. For example, the print job might require printing multiple halftones such as printing a coarse halftone for graphics and a fine halftone for photography. The magnitude of streaks is typically different for different halftones. Each halftone may have a different ERC. If only one halftone is monitored and the compensating TRCs are derived for only the monitored halftone, the compensation may not be adequate for other halftones.
In another approach, each halftone screen is printed and compensating ERC or TRC for each halftone is determined. However, since a typical printer prints a plurality of halftones, such approach is costly and time consuming in most applications.
There is a need for methods and apparatuses which overcome the aforementioned problems and others.